From MineScape Wiki

The Abyss is an area of Abyssal Space that contains rifts created by the Zamorak Magical Institute with which they directly access the runic altars without the need for a talisman, with the exception of the Astral and Wrath Altar. Accessing this area of the Abyss requires completion of the miniquest Enter the Abyss. In appearance, it seems to be oddly organic, with eyes, boils, and strange-looking tendrils.

The Abyss consists of two rings: a dangerous outer ring in a multicombat area and a safe inner ring with rifts leading to the runic altars. Aside from the aggressive abyssal creatures inhabiting it, players who enter the Abyss will have their prayer points drained to 0 and be immediately given a skull, although an abyssal bracelet can be worn to prevent the player from getting automatically skulled.

Perhaps the most dangerous aspect of accessing the Abyss is the fact that players have to travel through part of the low-level Wilderness to reach the Mage of Zamorak, who is the only NPC that teleports the player to this part of the Abyss. This causes a risk of being attacked by the occasional player killer, so some degree of caution is required when reaching the mage.